This invention relates to a two-wire loop electric circuit arrangement.
In telemetering or automatic control systems use is often made of a so-called 4-20 mA transmitter in combination with a two-wire loop, information being transmitted over the loop by analogue control of the current from the transmitter between the 4 mA and 20 mA limits. Such a transmitter can be considered to be a 4 mA constant current generator and a signal current generator providing a further 16 mA superimposed on the 4 mA.
In GB-A-1417292 there is disclosed such an arrangement in which a 4-20 mA transmitter is connected in the loop in series with a signal and power supply converter which utilises the 4 mA residual current in the loop, this being representative of a zero signal, to generate a supply voltage for a load, and which operates to convert any signal current above the 4 mA limit into a voltage proportional to that signal current, the signal voltage generated being supplied to the load. The load thus receives a power supply voltage and a signal voltage from the converter, both voltages being derived from the loop current from the 4-20 mA transmitter. The load can be any appropriate type of control, indicating or alarm circuit, or a signal conditioning unit.
Such a known arrangement has the advantage that no separate power supply is needed for the load.
However, in the known arrangement the supply voltage generation circuitry is connected in series with the signal conversion circuitry in the converter and this introduces an additional voltage drop into the loop. In many arrangements the available total loop driving voltage is limited, for safety or other reasons, and the additional voltage drop introduced must be substracted from that available to other devices in the loop.
Further, it is common practice to connect a diode in the loop either to provide protection against inadvertent polarity reversal, or as a test point for connection of, for example, an analogue moving coil meter. It would be desirable to connect a measuring instrument across such diode such that the loop current is diverted into the instrument, but this would place severe constraints on the voltage available to the instrument.